1. Field of the Invention
The present invention relates to a wire bonding apparatus for and a wire bonding method of connecting an electrode of a chip and an electrode of a substrate by a wire.
2. Description of the Prior Art
In the process of fabricating electronic parts, an electrode of a chip and an electrode of a substrate are connected to each other by a wire. Hereinbelow, the conventional wire bonding will be explained.
FIGS. 9A, 9B, 9C and 9D are side views each showing a main portion of a conventional wire bonding apparatus in operation, wherein these figures show states of the wire bonding apparatus in order of operations. First, in FIG. 9A, numeral 51 denotes a bonding arm, numeral 52 a capillary tool held at a tip portion of the bonding arm 51, and numeral 53 denotes a wire inserted through the capillary tool 52. The wire 53 is drawn out downward from a lower end of the capillary tool 52. This drawn-out portion is, in general, called a tail and represented by symbol 53a.
As shown in FIG. 9A, a torch (electrode) 54 is moved close to a lower end of the tail 53a. Subsequently, a high voltage is applied to the torch 54 to generate an electric spark between the torch 54 and the lower end of the tail 53a so that a ball 55 is formed at the lower end of the tail 53a. Thereafter, as shown in FIG. 9B, the capillary tool 52 is moved to a position just above an electrode (not shown) of a chip 56 and then lowered by pivoting the bonding arm 51. Accordingly, the ball 55 is pressed onto the electrode of the chip 56 by means of a lower end surface of the capillary tool 52 so as to be bonded thereto. Numeral 57 denotes a substrate on which the chip 56 is mounted. Numeral 58 denotes a support section for supporting the substrate 57. The substrate 57 is, in general in the form of a printed board or a lead frame.
Subsequently, as shown in FIG. 9C, the capillary tool 52 is moved to a position just above an electrode (not shown) of the substrate 57 and then lowered so that the wire 53 drawn out from its lower end is pressed onto the electrode of the substrate 57 so as to be bonded thereto. Thereafter, as shown in FIG. 9D, the capillary tool 52 is raised. When the capillary tool 52 is moved upward by a distance corresponding to the tail 53a, the wire 53 is clamped or held by a clamper 59 and then raised upward along with the capillary tool 52. As a result, the wire 53 is cut at a bonding point A on the substrate 57 so that the wire 53 drawn out from the lower end of the capillary tool 52 becomes a new tail 53a. Then, the process returns to the state shown in FIG. 9A and the foregoing bonding operations are repeated.
In the foregoing wire bonding, the process of forming the ball 55 at the lower end of the tail 53a by the electric spark is important. However, in the conventional wire bonding apparatus, failure in drawing out the tail 53a is liable to occur as described hereinbelow so that the ball 55 can not be formed frequently. Specifically, as described above and shown in FIG. 9C, the wire 53 is pressed onto the electrode of the substrate 57 by the lower end surface of the capillary tool 52 so as to be bonded thereto. However, at this instant, the wire 53 is liable to be cut due to the edge at the lower end of the capillary tool 52. Thus, even when the capillary tool 52 is raised for forming the tail 53a, the wire 53 is cut easily in an instant. Accordingly, in the worst case, as shown in FIG. 9D, a lower end portion of the wire 53, which should otherwise be the tail 53a, gets into the capillary tool 52 so that no tail is resulted.
FIG. 10 is a partial sectional view of the capillary tool 52 with no tail drawn out, which is caused in the conventional wire bonding apparatus. As shown in the figure, since the lower end of the wire 53 is received inside the capillary tool 52, even if the torch 54 is moved closer and applied with a high voltage, no spark is generated between the torch 54 and the lower end of the wire 53 so that the ball 55 is not formed. In this case, the bonding of the ball 55 onto the electrode of the chip 56 as shown in FIG. 9B can not be achieved.
Occurrence of no spark even upon application of the high voltage to the torch 54 is detected by known electric detection means and notified to an operator by a signaling element, such as a buzzer. On this occasion, the operator draws out the wire 53 from the inside of the capillary tool 52 using a pincette to provide the tail 53a, and then restarts the wire bonding operations.
However, a diameter of the capillary tool 52 is, in general, about 2 mm which is very small, and further, the wire 53 having a diameter of about 20 .mu.m, which is extra fine, is inserted through the inside of the capillary tool 52. Accordingly, it is quite troublesome to draw out the wire 53 from the inside of the capillary tool 52 using the pincette, which imposes much labor upon the operator. Further, as described above, failure in drawing out the tail 53a occurs with high frequency so that the operator is frequently required to stop the operation of the wire bonding apparatus and perform the wire drawing-out operations. Failure in formation of the ball 55 also occurs due to the tail 53a being too short, bent or the like.